The present invention relates to a method for identifying erroneous detection of a pre-pit synchronization bit at the time of detecting pre-pits from readout signals of an optical disc (disk) having the pre-pits formed therein in a given signal format and a method for identifying stability/instability of pre-pit synchronization bit detection by use of the identified result of the erroneous detection identifying method, as well as electric circuits for implementing these methods. In particular, the present invention concerns a technique for identifying the erroneous detection of a pre-pit synchronization bit and identifying the stability/instability of pre-pit synchronization bit detection with high accuracy.
Recordable optical discs, such as recordable CDs like CD-R and CD-RW discs and recordable DVDs like DVD-R, DVD+RW, DVD-RAM and DVD-RW, have recording tracks wobbling in predetermined cycles. Generally, during recording, a wobble signal is extracted out of a push-pull signal generated by receiving a reflection of a recording laser light beam off the optical discspecifically, the push-pull signal is indicative of a difference between outputs from light-receiving elements positioned symmetrically about the longitudinal axis of the recording track. Spindle motor control, reproduction of address information, generation of reference clock pulses for a recording signal, etc. are performed on the basis of the extracted wobble signal. Where the optical disc has pre-pits, i.e. marks previously formed intermittently along the recording track in a laterally displaced relation to the centerline of the track, pre-pit signals (i.e., signals indicative of detected pre-pits) are extracted out of the push-pull signal, and the generation of reference clock pulses for a recording signal, reproduction of address information, spindle motor control, etc. are performed on the basis of the extracted pre-pit signals.
Examples of the known pre-pit signal formats include one in which synchronization bits are placed at predetermined locations in every other one of a plurality of track-constituting unit sections arranged in the longitudinal direction of the track, and in which in each regions of the track where the synchronization bit substantially overlaps with the synchronization bit of an adjacent track in a radial direction of the optical disc, the recorded location of the synchronization bit is displaced by one track-constituting unit section in the longitudinal direction of the track. As a good example of such a pre-pit signal format, the pre-pit signal format of a DVD-R disc is explained below with reference to FIG. 2. As shown, the recording track (groove track) of the DVD-R disc wobbles at a predetermined frequency 140 kHz, and each length equal to eight waves of the wobbling recording track is set as a track-constituting unit section (sync frame) for recording information. 26 such sync frames constitute a sector and 16 sectors constitute an ECC block, so that information is recorded onto the optical disc ECC block by ECC block. The 26 sync frames constituting a sector consist of even-numbered frames (frame 0, frame 2, . . . , frame 24) and odd-numbered frames (frame 1, frame 3, . . . , frame 25). In a land track adjacent to the outer edge of the groove track, there are formed land pre-pits in such a manner that the land pre-pits occur once for every two sync frames in correspondence with the groove track. The land pre-pits are normally recorded in the even-numbered sync frames; however, in each region where the land pre-pits in the outer land track substantially overlap radially with similar land pre-pits in another land track adjacent to the inner edge of the groove track, the land pre-pits of the outer land track are formed in the odd-numbered sync frame immediately following the even-numbered sync frame in question (i.e., at a location displaced by one sync frame in the longitudinal direction of the track). so as to avoid undesired interference between the radially overlapping land pre-pits of the inner and outer land tracks (i.e., between radially adjoining land tracks). Each set of the land pre-pits thus formed on the land track consists of a total three bits (i.e., xe2x80x9cb2xe2x80x9d, xe2x80x9cb1xe2x80x9d and xe2x80x9cb0xe2x80x9d bits in the head-to-rear direction), each formed at one of predetermined phase angle locations of the leading wobble wave within the sync frame. In TABLE 1 below, there are shown pre-pit codes constituted by various combinations of logical values at the xe2x80x9cb2xe2x80x9d, xe2x80x9cb1xe2x80x9d and xe2x80x9cb0xe2x80x9d bits and pre-pit information represented by the pre-pit codes. Note that a logical value xe2x80x9c1xe2x80x9d of the bit indicates that a land pre-pit is formed at the corresponding location on the disc while a logical value xe2x80x9c0xe2x80x9d of the bit indicates that no land pre-pit is formed at the corresponding location on the disc.
The leading xe2x80x9cb2xe2x80x9d bit in the pre-pit code is a synchronization signal of the corresponding sync frame, which is invariably present at the head of each sync frame having pre-pits formed therein. The xe2x80x9cb1xe2x80x9d bit is a signal indicating whether the corresponding pre-pit information is a sector synchronization code or data (information such as position information), and this b1 bit takes the value xe2x80x9c1xe2x80x9d if the corresponding pre-pit information is a sector synchronization code but takes the value xe2x80x9c0xe2x80x9d if the corresponding pre-pit information is data. Further, in the case of the sector synchronization code, the xe2x80x9cb0xe2x80x9d bit is a bit signal indicating whether the recorded location of the pre-pits is in an even-numbered sync frame or in an odd-numbered sync frame. In the case of the data, the xe2x80x9cb0xe2x80x9d bit represents contents of the data, i.e. data of one bit constituting information such as position information. Where the sector synchronization code is recorded in an even-numbered sync frame, it is formed in the leading (i.e., 0th) sync frame of the sector, while the sector synchronization code is recorded in an odd-numbered sync frame, it is formed in the second-from-head (i.e., 1st) sync frame of the sector. Namely, in every sector on the track of the optical discsuch a sector synchronization code is formed in either the leading (0th) sync frame or the second-from-head (i.e., 1st) sync frame.
During recording or reproduction on the optical disc, pre-pit signal components can be determined as projecting beyond peak points of wobble signal components contained in a push-pull tracking error signal, as shown in FIG. 3. Thus, the pre-pit signal can be detected, for example, by comparing a tracking error signal with an appropriate threshold value, as also shown in FIG. 3. By decoding the thus-detected pre-pit signal, there can be obtained information, such as address information, sync frame synchronization information (b2=xe2x80x9c1xe2x80x9d) and sector synchronization information (b2, b1, b0=xe2x80x9c1, 1, 1xe2x80x9d or xe2x80x9c1, 1, 0xe2x80x9d).
As noted earlier, the synchronization bit of the pre-pit code is normally placed in every other track-constituting unit section appearing along the longitudinal direction of the track, it is recorded in a track-constituting unit section immediately following the normal track-constituting unit section in such a region of the track where the synchronization bit in the land track substantially overlap radially with the synchronization bit in an inner adjacent track. Thus, the synchronization bit sometimes occurs in two successive track-constituting unit sections or no synchronization bit sometimes occurs in two successive track-constituting unit sections. As a consequence, when the synchronization bit has been detected in two successive track-constituting unit sections, it is very difficult to determine whether the successive synchronization bits are correctly-detected synchronization bits, or erroneously-detected synchronization bits despite loss of the predetermined synchronization. Similarly, when no synchronization bit has been detected in two successive track-constituting unit sections, it is very difficult to determine whether the non-detection of the synchronization bit is due to the fact that the synchronization bit is just missing from the two successive track-constituting unit sections although the predetermined synchronization has been attained, or due to loss of the predetermined synchronization.
In view of the foregoing, it is an object of the present invention to provide a method capable of identifying, with high accuracy, erroneous detection of a pre-pit synchronization bit at the time of detecting pre-pit signals from readout signals of an optical disc having formed therein pre-pits in a given signal format, and a method capable of identifying, with high accuracy, stability/instability of pre-pit synchronization bit detection by use of the erroneous detection identifying method and an identified result of the erroneous detection identifying method. It is another object of the present invention to provide electric circuits suitable for implementing the above-mentioned methods of the present invention.
An optical disc has been known on which synchronization bits are recorded as pre-pits, at predetermined locations of a track, in every other one of a plurality of successive track-constituting unit sections provided along a longitudinal direction of the track and on which in each region of the track where the synchronization bit substantially overlaps with the synchronization bit of an adjacent track in a radial direction of the optical disc, a recorded location of the synchronization bit is displaced, in the longitudinal direction of the track, by one track-constituting unit section 1. In this type of optical disc, the region, where the recorded location of the synchronization bit is displaced by one track-constituting unit section, does not occur so frequently. Thus, the present invention is arranged to identify erroneous detection of a pre-pit synchronization bit, using such a characteristic of the optical disc.
Namely, at the time of detecting a pre-pit synchronization bit from a readout signal of the optical disc, the erroneous detection identifying method of the present invention performs: a step of detecting presence/absence of the synchronization bit at the predetermined location in each of the track-constituting unit sections; a step of, when the synchronization bit is detected or not detected in two or more successive track-constituting unit sections and then the synchronization bit is detected or not detected in following two successive track-constituting unit sections within a predetermined time, determining that detection or non-detection of the synchronization bit in the following two successive track-constituting unit sections is erroneous; and a step of, when the synchronization bit is detected only in every other track-constituting unit section within the predetermined time period and then the synchronization bit is detected in two successive track-constituting unit sections following the predetermined time period or when the synchronization bit is detected only in every other track-constituting unit section within the predetermined time period and then the synchronization bit is not detected in the two successive track-constituting unit sections following the predetermined time period, determining that detection or non-detection of the synchronization bit in the two successive track-constituting unit sections following the predetermined time period is accurate or has no error.
Thus, when the synchronization bit has been detected in two successive track-constituting unit sections, the present invention can appropriately determine whether the successive synchronization bits are correctly-detected synchronization bits, or erroneously-detected synchronization bits despite loss of the predetermined synchronization. Similarly, when the synchronization bit has not been detected in two successive track-constituting unit sections, the present invention can appropriately determine whether the non-detection of the synchronization bit is due to the fact that the synchronization bit is just missing from the two successive track-constituting unit sections although the predetermined synchronization has been achieved, or due to loss of the predetermined synchronization. With this arrangement, erroneous detection of the synchronization bit can be identified with high accuracy. In addition to the above-mentioned determination steps, the present invention may further comprise a determination step of, when the synchronization bit is detected in every other track-constituting unit sections, determining that detection or non-detection of the synchronization bit is accurate or has no error, but when the synchronization bit is detected or not detected in three or more successive track-constituting unit sections, determining that detection or non-detection of the synchronization bit in the three or more successive track-constituting unit sections is erroneous or has an error.
According to another aspect of the present invention, there is provided a method for identifying stability/instability of pre-pit synchronization bit detection from an optical disc, which comprises: a step of, on the basis of a determination result by the above-mentioned erroneous detection identifying method, determining present/absence of erroneous detection of a pre-pit synchronization bit in each of optionally-set time periods; a step of updating a count value by a predetermined amount in one counting direction each time no erroneous detection of the pre-pit synchronization bit is made in one of the time periods, but updating the count value by a predetermined amount in another counting direction each time erroneous detection of the pre-pit synchronization bit is made in one of the time periods; and a step of, when the count value has reached a predetermined value in the one counting direction, determining that the detection of the pre-pit synchronization bit is in a stable state, but when the count value has reached a predetermined value in the other counting direction, determining that the detection of the pre-pit synchronization bit is in an instable state.
Because the erroneous detection of the synchronization bit can be identified with high accuracy by the above-mentioned erroneous detection identifying method of the invention, the method for identifying stability/instability of pre-pit synchronization bit detection, using the erroneous detection identifying method, can also identify the stability/instability of pre-pit synchronization bit detection with high accuracy. Further, servo gains for tracking and focus control of the optical pickup and the like can be changed using an identified result of the stability/instability identification; for example, the servo gains may be increased when the detection of the pre-pit synchronization bit is determined as stable, and the servo gains may be decreased when the detection of the pre-pit synchronization bit is determined as instable. Furthermore, spindle motor control can be changed using the identified result of the stability/instability identification; for example, frequency control of the spindle motor may be employed when the detection of the pre-pit synchronization bit is determined as stable, and phase control of the spindle motor may be employed when the detection of the pre-pit synchronization bit is determined as instable.
According to still another embodiment of the present invention, there is provided a circuit for identifying erroneous detection of a pre-pit synchronization bit from a readout signal of an optical disc, which comprises: a timing generator section for instructing arrival timing of the synchronization bit for each of the track-constituting unit sections; a synchronization bit detector section for detecting presence/absence of the synchronization bit at the arrival timing instructed by the timing generator section; a storage section for storing data indicative of the presence/absence of the synchronization bit detected via the synchronization bit detector section for each of the track-constituting unit sections; and a decoder section for decoding stored data of the storage section for each of the track-constituting unit sections. When the synchronization bit is detected only in every other one of the track-constituting unit sections, the decoder section determines that detection of the synchronization bit in the plurality of track-constituting unit sections is accurate. When the synchronization bit is detected in two successive track-constituting unit sections only once but detected only in every other one of the track-constituting unit sections on other occasions, or when the synchronization bit is not detected in two successive track-constituting unit sections only once but detected only in every other one of the track-constituting unit sections on other occasions, the decoder section determines that the detection of the synchronization bit in the plurality of track-constituting unit sections is accurate, provided that the synchronization bit has been detected only in every other track-constituting unit section in a preceding predetermined time period. Further, when the synchronization bit has been detected or not detected in two or more successive track-constituting unit sections in the preceding predetermined time period, the decoder section determines that the detection of the synchronization bit in the plurality of track-constituting unit sections is erroneous. Furthermore, when the synchronization bit is detected or not detected in two successive track-constituting unit sections more than once, the decoder section determines that the detection of the synchronization bit in the plurality of track-constituting unit sections is erroneous. Moreover, when the synchronization bit is detected or not detected in three or more successive track-constituting unit sections, the decoder section determines that the detection of the synchronization bit in the plurality of track-constituting unit sections is erroneous.
According to still another embodiment of the present invention, there is provided a circuit for identifying stability/instability of pre-pit synchronization bit detection from an optical disc, which comprises: a counter for, on the basis of a determination output of the above-mentioned circuit for identifying erroneous detection of a pre-pit synchronization, updating a count value by a predetermined amount in one counting direction each time it has been determined that the detection of the pre-pit synchronization bit in one of the track-constituting unit sections is accurate, but updating the count value by a predetermined amount in another counting direction each time it has been determined that the detection of the pre-pit synchronization bit in one of the track-constituting unit sections is erroneous; and a determination section for, when the count value of the counter has reached a predetermined value in the one counting direction, determining that the detection of the pre-pit synchronization bit is in a stable state, but when the count value has reached a predetermined value in the other counting direction, determining that the detection of the pre-pit synchronization bit is in an instable state.